Dc-dc converter and display device including the same

ABSTRACT

A direct current-to-direct current (“DC-DC”) converter includes: a first converter which outputs a first power voltage in a normal mode or a power saving mode based on a inductor current generated therein, where the first converter operates in a first driving manner in the normal mode, and operates in a second driving manner in the power saving mode; a second converter which outputs a second power voltage based on a inductor current generated therein, where the second converter operates in a third driving manner in the power saving mode, and a magnitude of the second power voltage in the power saving mode is different from that in the normal mode; and a mode selector which supplies a mode control signal to the first and second converters, where the first and second converters are driven in the normal mode or the power saving mode based on the mode control signal.

This application is a continuation of U.S. patent application Ser. No.18/073,999, filed on Dec. 2, 2022, which is a continuation of U.S.patent application Ser. No. 17/397,053, filed on Aug. 9, 2021, which isa continuation of U.S. patent application Ser. No. 16/895,510, filed onJun. 8, 2020, which is a continuation of U.S. patent application Ser.No. 16/132,905, filed on Sep. 17, 2018, which claims priority to KoreanPatent Application No. 10-2017-0165314, filed on Dec. 4, 2017, and allthe benefits accruing therefrom under 35 U.S.C. § 119, the content ofwhich in its entirety is herein incorporated by reference.

BACKGROUND 1. Field

Embodiments of the disclosure relate to a display device, and moreparticularly, to a direct current-to-direct current (“DC-DC”) converterincluded in a display device.

2. Related Art

A display device typically includes a DC-DC converter for generating ahigh-potential voltage and a low-potential voltage, which are used todrive pixels, by converting an input voltage supplied from the outside.In such a display device, the DC-DC converter may generate and supplypositive and negative voltages to the pixels through a voltage line.

SUMMARY

As size or range of displayable luminance of a display panel included ina display device increases, the range of driving current provided to thedisplay panel increases. Accordingly, when the display device is drivenat a low luminance, etc., where the magnitude of driving current issmall, the conversion efficiency of a direct current-to-direct current(“DC-DC”) converter in the display device is decreased. In addition, aspower loss is increased due to a relatively large internal resistorincluded in the DC-DC converter or repeated switching of the DC-DCconverter, unnecessary power consumption and heat generation increase.

Embodiments provide a DC-DC converter that is driven in a normal modeand a power saving mode with reduced power loss.

Embodiments also provide a display device including the DC-DC converter.

According to an embodiment of the disclosure, a DC-DC converterincludes: a first converter which outputs a first power voltage in anormal mode or in a power saving mode based on a first inductor currentgenerated by alternately turning on a plurality of transistors therein,where the first converter operates in a first driving manner in thenormal mode, and operates in a second driving manner in the power savingmode; a second converter which outputs a second power voltage based on asecond inductor current generated by alternately turning on a pluralityof transistors therein, where the second converter operates in a thirddriving manner in the power saving mode, and a magnitude of the secondpower voltage in the power saving mode is different from a magnitude ofthe second power voltage in the normal mode; and a mode selector whichsupplies a mode control signal to the first and second converters, wherethe first and second converters are driven in the normal mode or thepower saving mode based on the mode control signal.

In an embodiment, turn-on times of the transistors in the second drivingmanner may be less than turn-on times of the transistors in the firstdriving manner during a same period, and turn-on times of thetransistors in the third driving manner may be smaller than turn-ontimes of the transistors in the second driving manner during the sameperiod.

In an embodiment, the second converter may operate in one of the firstdriving manner, the second driving manner and the third driving manner,which is determined based on the magnitude of a load of a display panelconnected thereto, in the normal mode.

In an embodiment, the third driving manner may be implemented as a pulsefrequency modulation (“PFM”) manner.

In an embodiment, when the first converter operates in the first drivingmanner, a magnitude of the first inductor current may be continuouslychanged, and when the first converter operates in the second drivingmanner, the magnitude of the first inductor current may not be changedduring a predetermined period.

In an embodiment, when the second converter operates in the seconddriving manner, a magnitude of the second inductor current may not bechanged during a first discontinuous period, and when the secondconverter operates in the third driving manner, m the magnitude of thesecond inductor current may not be changed during a second discontinuousperiod. In such an embodiment, the second discontinuous period may belonger than the first discontinuous period.

In an embodiment, the first inductor current generated in the firstdriving manner may have a first peak value, and the first inductorcurrent generated in the second driving manner may have a second peakvalue less than the first peak value.

In an embodiment, a driving frequency in the power saving mode may belower than a driving frequency in the normal mode.

In an embodiment, the driving frequency in the power saving mode may bedecreased to a preset value as a magnitude of a load of a display panelconnected thereto is decreased.

In an embodiment, the first converter may include: a first inductorcoupled between an input power source and a first node, where the firstinductor may generate the first inductor current; a first transistorcoupled between the first node and a ground; a first power savingtransistor coupled in parallel to the first transistor, where the firstpower saving transistor may have a size smaller than a size of the firsttransistor; a second transistor coupled between the first node and afirst output end; a second power saving transistor coupled in parallelto the second transistor, where the second power saving transistor mayhave a size smaller than a size of the second transistor; and a firstswitching controller which controls on/off operations of the firsttransistor, the second transistor, the first power saving transistor andthe second power saving transistor. In such an embodiment, the firsttransistor and the second transistor may be alternately turned on in thenormal mode, and the first power saving transistor and the second powersaving transistor may be alternately turned on in the power saving mode.

In an embodiment, the first converter may further include: a firstswitch coupled between a gate electrode of the first transistor and agate electrode of the first power saving transistor, where the firstswitch may be turned on in the normal mode; a second switch coupledbetween the gate electrode of the first power saving transistor and thefirst switching controller, where the second switch may be turned on inthe power saving mode; a third switch coupled between a gate electrodeof the second transistor and a gate electrode of the second power savingtransistor, where the third switch may be turned on in the normal mode;and a fourth switch coupled between the gate electrode of the secondpower saving transistor and the first switching controller, where thefourth switch may be turned on in the power saving mode.

In an embodiment, the first power saving transistor and the second powersaving transistor may be alternately turned on in the normal mode, inwhich the first and third switches are turned on.

In an embodiment, the second converter may include: a second inductorcoupled between a second node and a ground, where the second inductormay generate the second inductor current; a third transistor coupledbetween an input power source and the second node; a third power savingtransistor coupled in parallel to the third transistor, where the thirdpower saving transistor may have a size smaller than a size of the thirdtransistor; a fourth transistor coupled between the second node and asecond output end; a fourth power saving transistor coupled in parallelto the fourth transistor, where the fourth power saving transistor mayhave a size smaller than a size of the fourth transistor; and a secondswitching controller which controls on/off operations of the thirdtransistor, the fourth transistor, the third power saving transistor andthe fourth power saving transistor. In such an embodiment, the thirdtransistor and the fourth transistor may be alternately turned on in thenormal mode, and the third power saving transistor and the fourth powersaving transistor may be alternately turned on in the power saving mode.

In an embodiment, the second converter may further include: a fifthswitch coupled between a gate electrode of the third transistor and agate electrode of the third power saving transistor, where the fifthswitch may be turned on in the normal mode; a sixth switch coupledbetween the gate electrode of the third power saving transistor and thesecond switching controller, where the sixth switch may be turned on inthe power saving mode; a seventh switch coupled between a gate electrodeof the fourth transistor and a gate electrode of the fourth power savingtransistor, where the seventh switch may be turned on in the normalmode; and an eighth switch coupled between the gate electrode of thefourth power saving transistor and the second switching controller,where the eighth switch may be turned on in the power saving mode.

In an embodiment, the third power saving transistor and the fourth powersaving transistor may be alternately turned on in the normal mode, inwhich the fifth and seventh switches are turned on.

In an embodiment, the second converter may change the magnitude of thesecond power voltage while switching a driving manner thereof insynchronization with the mode control signal in a vertical blank period.

In an embodiment, the second converter may switch a driving mannerthereof in synchronization with the mode control signal in a verticalblank period, the magnitude of the second power voltage may be changedstep by step to a power saving mode target level throughout a pluralityof vertical blank periods before a driving mode is switched from thenormal mode to the power saving mode, and the magnitude of the secondpower voltage may be changed step by step to a normal mode target levelthroughout a plurality of vertical blank periods after the driving modeis switched from the power saving mode to the normal mode.

In an embodiment, the mode selector may allow the mode control signal tobe enabled in a black image period. In such an embodiment, the secondconverter may change the magnitude of the second power voltage in theblack image period, and switch a driving manner thereof insynchronization with the mode control signal.

In an embodiment, the DC-DC converter may further include a protectorwhich compares a variation in the first power voltage output from thefirst converter with a preset reference voltage during a black imageperiod in which a driving mode is switched the normal mode and the powersaving mode, and shuts down the DC-DC converter if the variation in thefirst power voltage is larger than the reference voltage.

In an embodiment, switching operations of the first and secondconverters may be stopped in the black image period. In such anembodiment, the protector may compare the variation in the first powervoltage with the reference voltage in a state in which an output end ofthe first converter and an output end of the second converter arefloated.

In an embodiment, the DC-DC converter may further include a protectorwhich compares the second power voltage output from the second converterwith a preset reference voltage during a black image period in which adriving mode is switched between the normal mode and the power savingmode, and shuts down the DC-DC converter if the magnitude of the secondpower voltage is greater than the reference voltage. In such anembodiment, the operation of the second converter may be stopped duringat least a portion of the black image period.

According to an embodiment of the disclosure, there is provided a DC-DCconverter including: a boost converter which outputs a first powervoltage in a normal mode or in a power saving mode based on a firstinductor current generated therein, where when the booster converter isin the power saving mode, the booster converter operates in a seconddriving manner, in which a magnitude of the first inductor current iscontinuously changed based on a first pulse width modulation (“PWM”)signal having a first driving frequency, and when the booster converteris in the power saving mode, the booster converter operates in a seconddriving manner in which the magnitude of the first inductor current isnot changed during a first discontinuous period based on the first PWMsignal having a second driving frequency; a first inverting converterwhich outputs a second power voltage based on a second inductor currentgenerated therein in the normal mode or in the power saving mode, wherewhen the first inverting converter is in the power saving mode, thefirst inverting converter operates in a third driving manner or a PFMmanner which lowers a frequency in the power saving mode, and when thefirst inverting convert is in the normal mode, the first invertingconverter operates in one of the first driving manner, the seconddriving manner and the third driving manner, which is determined basedon a magnitude of a load of a display panel, where when the firstinverting converts operates in the third driving manner, a magnitude ofthe second inductor current is not changed during a second discontinuousperiod based on a second PFM signal; and a second inverting converterwhich does not operate in the power saving mode, where the secondinverting converter outputs the second power voltage in the firstdriving manner or the second driving manner only when the load of thedisplay panel exceeds a preset reference load in the normal mode. Insuch an embodiment, the second discontinuous period is longer than thefirst discontinuous period.

In an embodiment, the second driving frequency may be lower than thefirst driving frequency.

In an embodiment, a magnitude of the second power voltage output fromthe first inverting converter in the power saving mode may be differentfrom a magnitude of the second power voltage output from the firstinverting converter in the normal mode.

According to an embodiment of the disclosure, a display device includes:a display panel including a plurality of pixels, where the display paneldisplays an image in one of a normal mode and a power saving mode; adata driver which provides a data signal to the display panel; and aDC-DC converter which supplies a first power voltage and a second powervoltage lower than the first power voltage to the display panel, andsupplies a source driving voltage to the data driver. In such anembodiment, the DC-DC converter includes: a first converter whichoutputs the first power voltage based on a first inductor currentgenerated by alternately turning on a plurality of transistors therein,where the first converter operates in a first driving manner in thenormal mode, and operates in a second driving manner in the power savingmode; and a second converter which outputs the second power voltagebased on a second inductor current generated by alternately turning on aplurality of transistors therein, where the second converter operates ina third driving manner in the power saving mode, and a magnitude of thesecond power voltage in the power saving mode is different from amagnitude of the second power voltage in the normal mode.

In an embodiment, turn-on times of the transistors in the second drivingmanner may be less than turn-on times of the transistors in the firstdriving manner during a same period, and turn-on times of thetransistors in the third driving manner may be less than turn-on timesof the transistors in the second driving manner during the same period.

In an embodiment, when the first converter operates in the first drivingmanner, the magnitude of the first inductor current may be continuouslychanged, and when the first converter operates the second drivingmanner, the magnitude of the first inductor current may not be changedduring a predetermined period.

In an embodiment, the second converter may operate in the second drivingmanner in the normal mode. In such an embodiment, when the secondconverter operates in the second driving manner, the magnitude of thesecond inductor current may not be changed during a first discontinuousperiod, and when the second converter operates in the third drivingmanner, the magnitude of the second inductor current may not be changedduring a second discontinuous period. In such an embodiment, the seconddiscontinuous period may be longer than the first discontinuous period.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the invention will become more apparentby describing in further detail exemplary embodiments thereof withreference to the accompanying drawings, in which:

FIG. 1 is a block diagram illustrating a display device according to anembodiment of the disclosure;

FIG. 2 is a block diagram illustrating a direct current-to-directcurrent (“DC-DC”) converter according to an embodiment of thedisclosure;

FIG. 3 is a diagram illustrating an embodiment of a first converterincluded in the DC-DC converter of FIG. 2 ;

FIG. 4 is a diagram schematically illustrating the sizes of transistorsincluded in the first converter of FIG. 3 ;

FIG. 5 is a diagram illustrating an exemplary embodiment of a secondconverter included in the DC-DC converter of FIG. 2 ;

FIGS. 6A to 6C are signal diagrams illustrating exemplary embodiments ofdriving manners in that the second converter of FIG. 5 generates aninductor current;

FIG. 7 is a diagram illustrating an alternative exemplary embodiment ofthe first converter included in the DC-DC converter of FIG. 2 ;

FIG. 8 is a diagram illustrating an alternative exemplary embodiment ofthe second converter included in the DC-DC converter of FIG. 2 ;

FIG. 9 is a waveform diagram illustrating an exemplary embodiment of anoperation of the DC-DC converter of FIG. 2 ;

FIG. 10 is a waveform diagram illustrating an alternative exemplaryembodiment of the operation of the DC-DC converter of FIG. 2 ;

FIG. 11 is a waveform diagram illustrating another alternative exemplaryembodiment of the operation of the DC-DC converter of FIG. 2 ;

FIG. 12 is a diagram illustrating a DC-DC converter according to analternative embodiment of the disclosure;

FIG. 13 is a waveform diagram illustrating an exemplary embodiment of anoperation of the DC-DC converter of FIG. 12 ;

FIGS. 14A and 14B are waveform diagrams illustrating alternativeexemplary embodiments of the operation of the DC-DC converter of FIG. 12;

FIG. 15 is a diagram illustrating a DC-DC converter according to anotheralternative embodiment of the disclosure; and

FIG. 16 is a diagram illustrating an exemplary embodiment of aninverting converter module included in the DC-DC converter of FIG. 15 .

DETAILED DESCRIPTION

The invention now will be described more fully hereinafter withreference to the accompanying drawings, in which various embodiments areshown. This invention may, however, be embodied in many different forms,and should not be construed as limited to the embodiments set forthherein. Rather, these embodiments are provided so that this disclosurewill be thorough and complete, and will fully convey the scope of theinvention to those skilled in the art. Like reference numerals refer tolike elements throughout.

It will be understood that when an element is referred to as being “on”another element, it can be directly on the other element or interveningelements may be present therebetween. In contrast, when an element isreferred to as being “directly on” another element, there are nointervening elements present.

It will be understood that, although the terms “first,” “second,”“third” etc. may be used herein to describe various elements,components, regions, layers and/or sections, these elements, components,regions, layers and/or sections should not be limited by these terms.These terms are only used to distinguish one element, component, region,layer or section from another element, component, region, layer orsection. Thus, “a first element,” “component,” “region,” “layer” or“section” discussed below could be termed a second element, component,region, layer or section without departing from the teachings herein.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting. As used herein, thesingular forms “a,” “an,” and “the” are intended to include the pluralforms, including “at least one,” unless the content clearly indicatesotherwise. “Or” means “and/or.” As used herein, the term “and/or”includes any and all combinations of one or more of the associatedlisted items. It will be further understood that the terms “comprises”and/or “comprising,” or “includes” and/or “including” when used in thisspecification, specify the presence of stated features, regions,integers, steps, operations, elements, and/or components, but do notpreclude the presence or addition of one or more other features,regions, integers, steps, operations, elements, components, and/orgroups thereof.

“About” or “approximately” as used herein is inclusive of the statedvalue and means within an acceptable range of deviation for theparticular value as determined by one of ordinary skill in the art,considering the measurement in question and the error associated withmeasurement of the particular quantity (i.e., the limitations of themeasurement system).

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art and thepresent disclosure, and will not be interpreted in an idealized oroverly formal sense unless expressly so defined herein.

Hereinafter, exemplary embodiments of the disclosure will be describedin detail with reference to the accompanying drawings.

FIG. 1 is a block diagram illustrating a display device according toembodiments of the disclosure.

Referring to FIG. 1 , the display device 1000 may include a directcurrent-to-direct current (“DC-DC”) converter 100, a display panel 300,and a data driver 500.

The display device 1000 may further include a scan driver (not shown)and a timing controller (not shown) for controlling operations of theDC-DC converter 100, the data driver 500 and the scan driver. In anembodiment, the timing controller and the data driver 500 may beintegrated in a single driver integrated circuit (“IC”) (referred to asD-IC in FIG. 1 ) or be directly disposed on the display panel 300. Thescan driver may be directly disposed on the display panel 300 or becoupled in an IC form to the display panel 300.

The display device 1000 may be an organic light emitting display device.Alternatively, the display device 1000 may be a liquid crystal displaydevice. However, this is merely illustrative, and the type of thedisplay device 1000 is not limited thereto.

In an embodiment, the display device 1000 may be driven in a normalmode, in which an image is normally displayed, and a power saving mode,in which an image is displayed with a low power. The power saving modeis a driving manner that minimizes power consumption by limiting themaximum luminance of the display panel 300 to a preset luminance orless. In one embodiment, for example, the power saving mode may be analways-on-display (“AOD”) mode in which simple display information isalways displayed, a predetermined display mode in which a screen isdisplayed at a very low luminance in a dark environment, or the like.

The DC-DC converter 100 may control or manage the magnitudes andsequences of source voltages ELVDD, ELVSS and AVDD provided to thedisplay panel 300 and the data driver 500, based on an input powersource VIN. The source voltages ELVDD, ELVSS and AVDD includes a firstpower voltage ELVDD and a second power voltage ELVSS, which are ahigh-potential voltage and a low-potential voltage, respectively, andused to drive pixels P, and a source driving voltage AVDD, which is avoltage used to drive the data driver 500. In an embodiment, the DC-DCconverter 100 may include a first converter 120 for converting a voltageof the input power source VIN into the first power voltage ELVDD, asecond converter 140 for converting the voltage of the input powersource VIN into the second power voltage ELVSS, and a third converter160 for converting the voltage of the input power source VIN into thesource driving voltage AVDD. The first and third converters 120 and 160may be boost converters, and the second converter 140 may be aninverting buck boost converter.

The first converter 120 may output the first power voltage ELVDD in afirst driving manner that generates a first inductor current by allowinga plurality of transistors to be alternately turned on in the normalmode. In an embodiment, the first driving manner may be a driving mannerin that the first inductor current (i.e., the magnitude of the firstinductor current) is continuously changed based on a first pulse widthmodulation (“PWM”) signal having a first driving frequency. In oneembodiment, for example, the first driving manner may be a continuousconduction mode (“CCM”) manner.

The first converter 120 may output the first power voltage ELVDD in asecond driving manner that generates the first inductor current withturn-on times (e.g., the number of turn-on periods per a unit period)less than those in the first driving manner, in the power saving mode.In an embodiment, the second driving manner may include a firstdiscontinuous period that is a period in which the first inductorcurrent (i.e., the magnitude of the first inductor current) is notchanged based on the first PWM signal having a second driving frequency.In one embodiment, for example, the second driving manner may be adiscontinuous conduction mode (“DOM”) manner. In an embodiment, thesecond driving frequency may be equal to or lower than the first drivingfrequency.

In an embodiment, the first inductor current generated in the firstdriving manner may have a first peak value, and the first inductorcurrent generated in the second driving manner may have a second peakvalue less than the first peak value.

The period in which the magnitude of the first inductor current is notchanged in the power saving mode corresponds to a period in which theswitching operation of transistors included in the first converter 120is stopped. In such an embodiment, the first discontinuous period maycorrespond to a period in which all the transistors included in thefirst converter 120 are in a turn-off state.

In such an embodiment, the first converter 120 may output the firstpower voltage ELVDD while continuously changing the magnitude of thefirst inductor current in the first driving manner, and output the firstpower voltage ELVDD while including a period in which the magnitude ofthe first inductor current is not changed in the second driving manner.Thus, the power consumption caused by the switching may be reduced dueto the period in which the magnitude of the first inductor current isnot changed in the power saving mode.

In the power saving mode, the second converter 140 may output the secondpower voltage ELVSS in a third driving manner. The third driving mannermay generate a second inductor current (i.e., a current in the secondconverter 140) with turn-on times less than those in the second drivingmanner. In an embodiment, the second converter 140 may output the secondpower voltage ELVSS in the third driving manner that includes a seconddiscontinuous period in which the magnitude of the second inductorcurrent is not changed based on a second PWM signal. The seconddiscontinuous period may correspond to a period in which transistorsincluded in the second converter 140 are all in the turn-off state. Inone embodiment, for example, the third driving manner may be a pulseskip mode (“PSM”) manner.

Alternatively, the third driving manner may decrease the turn-on times(switching times) of the transistors by lowering a frequency, using apulse frequency modulation (“PFM”) manner. In one embodiment, forexample, in the power saving mode, the switching times of thetransistors included in the second converter 140 may be decreased bylowering the frequency of a signal for driving the transistors, and theabsolute value of the second power voltage ELVSS may be decreased.

In such an embodiment, the second discontinuous period may be longerthan the first discontinuous period. Thus, the turn-on times (switchingtimes) of the transistors included in the second converter 140 in thethird driving manner is much less than those in the second drivingmanner. Accordingly, the power consumption of the second converter inthe power saving mode may also be reduced.

In an embodiment, in the normal mode, the second converter 140 may bedriven in one of the first to third driving manners based on themagnitude of a load (e.g., an emission luminance) of the display panel300. The second converter 140 may also adjust the magnitude of thesecond power voltage ELVSS according to the first to third drivingmanners. In one embodiment, for example, when the second power voltageELVSS is a negative voltage, the second power voltage ELVSS output inthe third driving manner may be greater than that output in the firstdriving manner. Therefore, the potential difference between the firstpower voltage ELVDD and the second power voltage ELVSS in the powersaving mode may be less than that in the normal mode.

The third converter 160 may operate in the same manner as the secondconverter 140. In one embodiment, for example, in the normal mode, thethird converter 160 may be driven in one of the first to third drivingmanners based on the magnitude of a load of the display panel 300. Inthe power saving mode, the third converter 160 may output the sourcedriving voltage AVDD in the third driving manner or in the PFM mannerthat lowers the frequency.

The display panel 300 displays an image. The display panel 300 mayinclude a plurality of pixels P that operate based on the first powervoltage ELVDD, the second power voltage ELVSS, a data signal, a scansignal and the like supplied thereto. In an embodiment, the second powervoltage ELVSS may be less (or lower) than the first power voltage ELVDD.In one embodiment, for example, the first power voltage ELVDD may be apositive voltage, and the second power voltage ELVSS may be a negativevoltage.

The data driver 500 may supply a data signal to the display panel.

In an embodiment, as described above, the display device 1000 and theDC-DC converter 100 included therein are driven in a driving manner thatreduces switching loss of the transistors included in the first to thirdconverters 120, 140 and 160 in the power saving mode, so that the powerconsumption in the power saving mode may be further reduced.

FIG. 2 is a block diagram illustrating a DC-DC converter according to anembodiment of the disclosure.

Referring to FIG. 2 , an embodiment of the DC-DC converter 100 mayinclude a first converter 120, a second converter 140 and a modeselector 170. For convenience of illustration and description, the thirdconverter 160 of FIG. 1 is not shown in FIG. 2 , and detaileddescription thereof will be omitted.

In such an embodiment, the DC-DC converter 100 may operate in one of anormal mode, in which a normal image is displayed, and a power savingmode, in which the displayable maximum luminance is limited, based on acontrol signal received from the outside.

The first converter 120 may output a first power voltage ELVDD, based onthe voltage of an input power source VIN and a mode control signalPS_EN. In the normal mode, the first converter 120 may output the firstpower voltage ELVDD in a first driving manner that generates a firstinductor current by allowing a plurality of transistors to bealternately turned on. In an embodiment, the first converter 120 mayoutput the first power voltage ELVDD in the first driving manner in thatthe first inductor current is continuously changed based on a first PWMsignal having a first driving frequency. The first inductor current is acurrent flowing through an inductor included in the first converter 120to determine the first power voltage ELVDD. In one embodiment, forexample, the first driving manner may be a CCM manner. The firstconverter 120 may be driven in only the CCM manner regardless of theload of the display panel in the normal mode to achieve the outputstability of the first power voltage ELVDD, which has great influence onpixel emission.

In the power saving mode, the first converter 120 may output the firstpower voltage ELVDD in a second driving manner that generates the firstinductor current with turn-on times less than those in the first drivingmanner. In an embodiment, the second driving manner may include a firstdiscontinuous period that is a period in which the first inductorcurrent is not changed based on the first PWM signal having a seconddriving frequency lower than the first driving frequency. In oneembodiment, for example, the second driving manner may be a DCM manner.

The second converter 140 may output a second power voltage ELVSS, basedon the voltage of the input power source VIN and the mode control signalPS_EN. In the power saving mode, the second converter 140 may output thesecond power voltage ELVSS in a third driving manner that generates asecond inductor current with turn-on times less than those in the seconddriving manner. In an embodiment, the second converter 140 may outputthe second power voltage ELVSS in the third driving manner including asecond discontinuous period in which the second inductor current is notchanged based on a second PWM signal. The second discontinuous period islonger than the first discontinuous period.

In one embodiment, for example, the third driving manner may beimplemented as a PSM manner or a PFM manner.

As the period in which the magnitude of the second inductor current isnot changed (i.e., the discontinuous period) is lengthened, themagnitude of the second power voltage ELVSS caused by the secondinductor current may be changed step by step. Therefore, the magnitudeof the second power voltage ELVSS in the normal mode may be differentfrom that of the second power voltage ELVSS in the power saving mode.

In the normal mode, the second converter 140 may be driven in one of thefirst driving manner, the second driving manner and the third drivingmanner according to the magnitude of a load of the display panel. In oneembodiment, for example, the second converter 140 may be driven in thefirst driving manner for high-luminance emission (when the load islarge). In such an embodiment, the second converter 140 may be driven inthe second driving manner for middle-luminance emission, and the secondconverter 140 may be driven in the third driving manner forlow-luminance emission.

In an embodiment, the driving frequency of the first converter 120and/or the second converter 140 in the normal mode may be equal to thatof the first converter 120 and/or the second converter 140 in the powersaving mode. Here, the driving frequency may be a frequency of controlsignals supplied to gate electrodes of the transistors of the firstconverter 120 and/or the second converter 140. That is, the switching(turn-on) and inductor current of the transistors may be controlledbased on the driving frequency. In one embodiment, for example, thefirst power voltage ELVDD may be output in the CCM manner of about 1.5megahertz (MHz) in the normal mode, and output the first power voltageELVDD in the CDM manner of about 1.5 MHz in the power saving mode. Insuch an embodiment, since the length of a period, in which thetransistors simultaneously have the turn-off state in the DCM manner, islonger than that of a period, in which the transistors simultaneouslyhave the turn-off state in the CCM manner, in one period having the samefrequency, the magnitude of an output voltage becomes smaller, and thepower consumption in the power saving mode is less than that in thenormal mode.

In an alternative embodiment, the driving frequency in the power savingmode may be smaller than that in the normal mode. In one embodiment, forexample, the first converter 120 may output the first power voltageELVDD in the CCM manner of about 1.5 MHz in the normal mode, and outputthe first power voltage ELVDD in the DCM manner of about 500 kilohertz(KHz) in the power saving mode. In such an embodiment, since the periodin which the transistors simultaneously have the turn-off sate is longerthan that in which the transistors are driven at the same drivingfrequency, the power consumption in the power saving mode may be lessthan that in the normal mode.

In an embodiment, the driving frequency in the power saving mode may bedecreased to a preset value as the magnitude of the load of the displaypanel is decreased. In one embodiment, for example, as the magnitude ofthe load of the display panel is decreased, the driving frequency may beselected in an order of about 500 KHz, about 400 KHz, about 300 KHz,about 200 KHz, and 100 KHz.

In such an embodiment, the DC-DC converter 100 directly controls thesecond power voltage ELVSS in the power saving mode, such that thesetting of the second power voltage ELVSS is relatively free. Thus, theluminance in the power saving mode having a level higher than that ofthe existing power saving mode may be implemented under an externalenvironment in which ambient illumination is bright. In such anembodiment, the driving manner of the DC-DC converter 100 and/or theadjustment of the magnitude of the second power voltage ELVSS may berelatively easily performed. Accordingly, the maximum luminance in thepower saving mode may be flexibly controlled.

The mode selector 170 may provide the mode control signal PS_EN to thefirst and second converters 120 and 140 such that the first and secondconverters 120 and 140 are driven in the normal mode or the power savingmode, based on an external command CMD. In one embodiment, for example,a period in which the mode control signal PS_EN is enabled maycorrespond to the power saving mode, and a period in which the modecontrol signal PS_EN is disabled may correspond to the normal mode. Oneof the first to third driving manners may be selected based on the modecontrol signal PS_EN.

In an embodiment, the mode selector 170 may include a load detector fordetecting a load of the display panel. Therefore, in the normal mode,the driving manner may be differently selected based on the load of thedisplay panel.

In an embodiment, in the power saving mode, operations of somecomponents included in the DC-DC converter 100 may be wholly orpartially stopped. In one embodiment, for example, operations of acurrent sensing block, an input/output voltage detection/stabilizationblock, a PWM/segment comparison block and the like, which are includedin the first converter 120 and/or the second converter 140, may bewholly or partially stopped. Accordingly, the supply of power to variousfunctional blocks that are not used in the power saving mode isinterrupted, so that unnecessary power consumption may be reduced.

In an embodiment, as described above, the DC-DC converter 100 adaptivelyselects driving manners of the first converter 120 and the secondconverter 140 based on the driving mode and the load of the displaypanel, so that power consumption may be reduced.

FIG. 3 is a diagram illustrating an exemplary embodiment of the firstconverter included in the DC-DC converter of FIG. 2 . FIG. 4 is adiagram schematically illustrating the sizes of transistors included inthe first converter of FIG. 3 .

Referring to FIGS. 2 to 4 , the first converter 120 may include a switchunit and a first switching controller 125. The switch unit may include afirst inductor L1, a first transistor M1, a first power savingtransistor PSM1, a second transistor M2, and a second power savingtransistor PSM2.

The first converter 120 may output the first power voltage ELVDD byconverting the input power source VIN, based on the mode control signalPS_EN. In one embodiment, for example, the first converter 120 mayinclude a boost converter.

The first inductor L1 may be coupled between an input end to which thevoltage of the input power source VIN is applied thereto and a firstnode N1. The first power voltage ELVDD may be controlled based on afirst inductor current flowing through the first inductor L1.

The first transistor M1 may be coupled between the first node and aground. The first transistor M1 may receive a first control signal G1applied from the first switching controller 125 to be turned on, andcontrol a current to flow through the first inductor L1.

The first power saving transistor PSM1 may be coupled in parallel to thefirst transistor M1. The first power saving transistor PSM1 may receivea first power saving control signal G11 applied from the first switchingcontroller 125 to be turned on, and control a current to flow throughthe first inductor L1. The first power saving transistor PSM1 mayperform the substantially same operation as the first transistor M1.

In an embodiment, as shown in FIG. 4 , the first power saving transistorPSM1 may have a size smaller than that of the first transistor M1. Insuch an embodiment, the allowable current amount of the first powersaving transistor PSM1 may be less than that of the first transistor M1.

The second transistor M2 may be coupled between the first node N1 and afirst output end from which the first power voltage ELVDD is output. Inthe normal mode, the second transistor M2 may be turned on alternatelywith the first transistor M1. Therefore, after an electromotive force isgenerated in the first inductor L1 as the first transistor M1 is turnedon, the second transistor M2 is turned on, so that a voltage V1 of thefirst node N1 may be converted into the first power voltage ELVDD basedon the input power source VIN, and the first power voltage ELVDD may beoutput to the first output end. The second transistor M2 may receive asecond control signal G2 applied from the first switching controller 125to be turned on. The first output end may be connected to a firstcapacitor C1.

The second power saving transistor PSM2 may be coupled in parallel tothe second transistor M2. The second power saving transistor PSM2 mayreceive a second power saving control signal G22 applied from the firstswitching controller 125 to be turned on. The second power savingtransistor PSM2 may be turned on alternately with the first power savingtransistor PSM1. The second power saving transistor PSM2 may perform thesubstantially same operation as the second transistor M2.

In such an embodiment, as shown in FIG. 4 , the second power savingtransistors PSM2 may have a size smaller than that of the secondtransistor M2.

In an embodiment, the first and second transistors M1 and M2 may operate(e.g., perform a switching operation) only in the normal mode, and thefirst and second power saving transistors PSM1 and PSM2 may operate onlyin the power saving mode. In one embodiment, for example, in the powersaving mode, the turn-off state of the first and second transistors M1and M2 may be maintained by the first and second control signals G1 andG2. In such an embodiment, in the normal mode, the turn-off state of thefirst and second power saving transistors PSM1 and PSM2 may bemaintained by the first and second power saving control signals G11 andG22.

In an alternative embodiment, the first and second transistors M1 and M2may operate only in the normal mode, and the first and second powersaving transistors PSM1 and PSM2 may operate in both of the normal modeand the power saving mode. In one embodiment, for example, the turn-offstate of the first transistor M1 may be maintained by the first controlsignal G1 in the normal mode, and the first power saving transistor PSM1may perform a switching operation in both of the normal mode and thepower saving mode in response to the first power saving control signalG11. In such an embodiment, the turn-off state of the second transistorM2 may be maintained by the second control signal G2, and the secondpower saving transistor PSM2 may perform a switching operation in bothof the normal mode and the power saving mode in response to the secondpower saving control signal G22.

When a transistor is switching, power loss occurs due to a parasiticcapacitance between electrodes of the transistor. The parasiticcapacitance may increase as the size of the transistor increase, and theamount of power loss may also increase as the parasitic capacitanceincreases. Thus, power consumption may be reduced by switching only thefirst and second power saving transistors PSM1 and PSM2 having smallsizes in the power saving mode for power saving.

In one embodiment, for example, as shown in FIG. 4 , the first powersaving transistor PSM1 may have a size smaller than that of the firsttransistor M1. In one embodiment, for example, the channel width and/orchannel length of the first power saving transistor PSM1 may be smallerthan the channel width and/or channel length of the first transistor M1.In an embodiment, the first transistor M1 and the first power savingtransistor PSM1 may be n-channel metal oxide semiconductor (“NMOS”)transistors.

The second power saving transistor PSM2 may have a size smaller thanthat of the second transistor M2. In one embodiment, for example, thechannel width and/or channel length of the second power savingtransistor PSM2 may be smaller than the channel width and/or channellength of the second transistor M2. In an embodiment, the secondtransistor M2 and the second power saving transistor PSM2 may bep-channel metal oxide semiconductor (“PMOS”) transistors.

In one embodiment, for example, the first and second transistors M1 andM2 allow a current of about 600 milliampere (mA) or more to cover up toa luminance of about 750 candela per square meter (cd/m²) to about 800cd/m². However, in an alternative embodiment, the first and secondtransistors M1 and M2 may cover only a luminance of about 100 cd/m² orless.

In an embodiment, the first converter 120 may output the first powervoltage ELVDD in the CCM manner (i.e., the first driving manner) usingthe first and second transistors M1 and M2 in the normal mode, andoutput the first power voltage ELVDD in the DCM manner (i.e., the seconddriving manner) using the first and second power saving transistors PSM1and PSM2 in the power saving mode.

The first switching controller 125 may control the on/off of the firsttransistor M1, the second transistor M2, the first power savingtransistor PSM1 and the second power saving transistor PSM2. The firstand second transistors M1 and M2 may be alternately turned on/off underthe control of the first switching controller 125. The first and secondpower saving transistors PSM1 and PSM2 may be alternately turned on/offunder the control of the first switching controller 125.

In an embodiment, the first switching controller 125 may set drivingfrequencies in the normal mode and the power saving mode to be differentfrom each other. In one embodiment, for example, the first switchingcontroller 125 may control the transistors M1, M2, PSM1 and PSM2 at adriving frequency of about 1.5 MHz in the normal mode, and control thetransistors M1, M2, PSM1 and PSM2 at a driving frequency of about 500KHz in the power saving mode.

In one embodiment, for example, the first switching controller 125 maygenerate a PWM signal having a predetermined frequency to control thedriving frequency of each of the control signals G1, G11, G2, and G22.The PWM signal may be a square wave signal. The generation of the PWMsignal and the control of the driving frequency may be performed byvarious methods known in the art.

In an embodiment, the first driving frequency in the power saving modemay be decreased to a preset value as the magnitude of the load of thedisplay panel is decreased. Thus, as the load of the display panel isdecreased, the switching times of the first and second power savingtransistors PSM1 and PSM2 may be decreased. Accordingly, the power losscaused by transistor switching may be reduced.

FIG. 5 is a diagram illustrating an exemplary embodiment of the secondconverter included in the DC-DC converter of FIG. 2 .

Referring to FIGS. 2 and 5 , the second converter 140 may include aswitch unit and a second switching controller 145. The switch unit mayinclude a second inductor L2, a third transistor M3, a third powersaving transistor PSM3, a fourth transistor M4, and a fourth powersaving transistor PSM4.

The second converter 140 may output the second power voltage ELVSS byconverting the input power source VIN, based on the mode control signalPS_EN. In one embodiment, for example, the second converter 140 mayinclude an inverting buck boost converter.

The second inductor L2 may be coupled between a second node N2 and aground. The second power voltage ELVSS may be controlled based on asecond inductor current flowing through the second inductor L2.

The third transistor M3 may be coupled between the input power sourceVIN and the second node N2. The third transistor M3 may receive a thirdcontrol signal G3 applied from the second switching controller 145 to beturned on, and control a current to flow through the second inductor L2.

The third power saving transistor PSM3 may be coupled in parallel to thethird transistor M3. The third power saving transistor PSM3 may receivea third power saving control signal G33 applied from the secondswitching controller 145 to be turned on, and control a current to flowthrough the second inductor L2. The third power saving transistor PSM3may perform the substantially same operation as the third transistor M3.

The third power saving transistor PSM3 may have a size smaller than thatof the third transistor M3. In such an embodiment, the allowable currentamount of the third power saving transistor PSM3 may be smaller thanthat of the third transistor M3.

The fourth transistor M4 may be coupled between the second node N2 and asecond output end. The fourth transistor M4 may be turned alternatelywith the third transistor M3 in response to a fourth control signal G4supplied from the second switching controller 145. Therefore, after anelectromotive force is generated in the second inductor L2 as the thirdtransistor M3 is turned on, the fourth transistor M4 is turned on, sothat a voltage V2 of the second node N2 may be converted into the secondpower voltage ELVSS based on the input power source VIN, and the secondpower voltage ELVSS may be output to the second output end. At thistime, the second node N2 may be defined as a common node of the thirdtransistor M3, the fourth transistor M4, and the second inductor L2. Inan embodiment, the second output end may be connected to a secondcapacitor C2.

The fourth power saving transistor PSM4 may be coupled in parallel tothe fourth transistor M4. The fourth power saving transistor PSM4 mayreceive a fourth power saving control signal G44 applied from the secondswitching controller 145 to be turned on. The fourth power savingtransistor PSM4 may be turned on alternately with the fourth transistorM4. The fourth power saving transistor PSM4 may perform thesubstantially same operation as the fourth transistor M4.

The fourth power saving transistor PSM4 may have a size smaller thanthat of the fourth transistor M4.

In an embodiment, the third and fourth transistors M3 and M4 may operate(e.g. perform a switching operation) only in the normal mode, and thethird and fourth power saving transistors PSM3 and PSM4 may operate onlyin the power saving mode. In one embodiment, for example, in the powersaving mode, the turn-off state of the third and fourth transistors M3and M4 may be maintained by the third and fourth control signals G3 andG4. In such an embodiment, in the normal mode, the turn-off state of thethird and fourth power saving transistors PSM3 and PSM4 may bemaintained by the third and fourth power saving control signals G33 andG44.

In an alternative embodiment, the third and fourth transistors M3 and M4may operate only in the normal mode, and the third and fourth powersaving transistors PSM3 and PSM4 may be operated in both of the normalmode and the power saving mode. In one embodiment, for example, theturn-off state of the third transistor M3 may be maintained by the thirdcontrol signal G3 in the normal mode, and the third power savingtransistor PSM3 may perform a switching operation in both of the normalmode and the power saving mode in response to the third power savingcontrol signal G33. In such an embodiment, the turn-off state of thefourth transistor M4 may be maintained by the fourth control signal G4,and the fourth power saving transistor PSM4 may perform a switchingoperation in both of the normal mode and the power saving mode inresponse to the fourth power saving control signal G44.

In an embodiment, the second converter 140 may output the second powervoltage ELVSS by selecting one of the first to third driving mannersusing the third and fourth transistors M3 and M4 in the normal mode, andoutput the second power voltage ELVSS in the third driving manner usingthe third and fourth power saving transistors PSM3 and PSM4 in the powersaving mode. Here, the first driving manner may be the CCM manner, andthe second driving manner may be the DCM manner. The third drivingmanner may be the PSM manner or the PFM manner.

The driving manner of the second converter 140 in the normal mode may beselectively used based on the load of the display panel. In the case ofhigh-luminance driving in which the load of the display panel is large,the second converter 140 may output the second power voltage ELVSS inthe first driving manner (e.g., the CCM manner). In the case ofmiddle-luminance driving in which the load of the display panel issmaller than a preset reference load, the second converter 140 mayoutput the second power voltage ELVSS in the second driving manner(e.g., the DCM manner). In the case of low-luminance driving in whichthe load of the display panel is smaller than the preset reference load,the second converter 140 may output the second power voltage ELVSS inthe third driving manner (e.g., the CCM manner).

The second switching controller 145 may control the on/off of the thirdtransistor M3, the fourth transistor M4, the third power savingtransistor PSM3 and the fourth power saving transistor PSM4. The thirdand fourth transistors M3 and M4 may be alternately turned on/off underthe control of the second switching controller 145. The third and fourthpower saving transistors PSM3 and PSM4 may be alternately turned on/offunder the control of the second switching controller 145.

In an embodiment, the second switching controller 145 may set drivingfrequencies in the normal mode and the power saving mode to be differentfrom each other. In such an embodiment, the first driving frequency inthe power saving mode may be decreased to a preset value as themagnitude of the load of the display panel is decreased. The switchingtimes (turn-on times per a unit period) of the transistors M3, M4, PSM3and PSM4 in the power saving mode are decreased by the second switchingcontroller 145, so that the power loss caused by transistor switchingmay be reduced.

FIGS. 6A to 6C are signal diagrams illustrating exemplary embodiments ofdriving manners in that the second converter of FIG. 5 generates aninductor current.

Referring to FIGS. 2, 3 and 6A to 6C, the first converter 120 mayoperate in the first driving manner in the normal mode, and operate inthe second driving manner in the power saving mode. The second converter140 may operate in one of the first to third driving manners in thenormal mode, and operate in the third driving manner in the power savingmode.

In FIGS. 6A to 6C, an embodiment in which the first converter 120 ofFIG. 3 operates in the first to third driving manners will be described.The second converter 140 may output the second power voltage ELVSSthrough the substantially same switching operation as the firstconverter 120.

The first driving manner may generate a first inductor current IL byalternately turning on the first and second transistors M1 and M2. In anembodiment, as shown in FIG. 6A, on/off operations of the first andsecond transistors M1 and M2 in a predetermined switching period T maybe repeated. In one embodiment, for example, in the switching period T,the turn-on state period of the first transistor M1 and the turn-onstate period of the second transistor M2 do not overlap with each other.

When the first transistor M1 is turned on (t1), the voltage V1 of thefirst node N1 may have a ground level GND, and the magnitude of thefirst inductor current IL may be increased by the difference between thevoltage of the input end and the voltage V1 of the first node N1.

When the first transistor M1 is turned off and the second transistor M2is turned on (t2), the voltage V1 of the first node N1 may be increasedto have the level of the first power voltage ELVDD, and the magnitude ofthe first inductor current IL may be decreased by the difference betweenthe voltage of the input end and the voltage V1 of the first node N1.

As the switching period is repeated, the magnitude of the first inductorcurrent IL may be continuously changed. In one embodiment, for example,the first driving manner may be the CCM manner. Since the first drivingmanner minimizes output ripples, the first driving manner has highoutput stability.

In an embodiment, as shown in FIG. 6B, the second driving manner mayinclude a period in which the first and second transistors M1 and M2 aresimultaneously turned off in one switching period T (hereinafter,referred to as a first discontinuous period t3). At this time, thevoltage V1 of the first node N1 may maintain the voltage of the inputpower source VIN. Since one end of the first inductor L1 is opened, thecurrent maintains a ground level GND, and the first inductor current ILis not changed during the first discontinuous period t3. In oneembodiment, for example, the peak value of the first inductor current ILin the second driving manner may be smaller than that of the firstinductor current IL in the first driving manner. In one embodiment, forexample, the first driving manner may be the DCM manner.

The first converter 120 may adjust the driving frequency in the seconddriving manner according to the load of the display panel. As themagnitude of the driving frequency is decreased based on the same time,the switching times (turn-on times) of the first and second transistorsM1 and M2 may be decreased. Thus, the power loss caused by the parasiticcapacitance due to the switching operations of the first and secondtransistors M1 and M2 may be reduced.

As shown in FIG. 6C, the third driving manner may include a seconddiscontinuous period t4 in which the first and second transistors M1 andM2 are simultaneously turned off in one switching period. The seconddiscontinuous period t4 may be longer than the first discontinuousperiod t3 in the second driving manner. In an embodiment, the thirddriving manner may skip a partial switching period. In such anembodiment, the switching of the first and second transistors M1 and M2may be skipped, and the first inductor current IL may not flow.Therefore, the amplitude of the first inductor current IL in the thirddriving manner may be smaller than that of the first inductor current ILin the second driving manner. In one embodiment, for example, the thirddriving manner may be the PSM manner.

Since the third driving manner skips the switching operation of thefirst and second transistors M1 and M2 in a predetermined period, thetotal switching times (turn-on times) may be decreased. Thus, the powerloss caused by the parasitic capacitance due to the switching operationsof the first and second transistors M1 and M2 may be reduced.

The operation of the second converter 140 is substantially identical tothe above-described operations, and any repetitive detailed descriptionthereof will be omitted.

FIG. 7 is a diagram illustrating an alternative exemplary of the firstconverter included in the DC-DC converter of FIG. 2 .

The first converter 120A of FIG. 7 may be substantially the same as thefirst converter of FIG. 3 , except for first to fourth switches S1 toS4. In FIG. 7 , components identical to those described with referenceto FIG. 3 are designated by like reference numerals, and any repetitivedetailed description thereof will be omitted.

Referring to FIGS. 3 and 7 , an embodiment of the first converter 120Amay include a first inductor L1, a first transistor M1, a first powersaving transistor PSM1, a second transistor M2, a second power savingtransistor PSM2, first to fourth switches S1, S2, S3 and S4, and a firstswitching controller 125A.

The first inductor L1 may be coupled between an input end to which thevoltage of the input power source VIN is applied thereto and a firstnode N1. The first transistor M1 may be coupled between the first nodeN1 and a ground. The first power saving transistor PSM1 may be coupledin parallel to the first transistor M1. The second transistor M2 may becoupled between the first node N1 and a first output end from which thefirst power voltage ELVDD is output. The second power saving transistorPSM2 may be coupled in parallel to the second transistor M2.

In such an embodiment, the size of the first power saving transistorPSM1 may be smaller than that of the first transistor M1, and the sizeof the second power saving transistor PSM2 may be smaller than that ofthe second transistor M2.

The first switch S1 may be coupled between a gate electrode of the firsttransistor M1 and a gate electrode of the first power saving transistorPSM1. The first switch S1 may be turned on in the normal mode inresponse to a control signal supplied from the first switchingcontroller 125A. The first switch S1 is in a turn-off state in the powersaving mode. Accordingly, both of the first transistor M1 and the firstpower saving transistor PSM1 may perform a switching operation in thenormal mode.

The second switch S2 may be coupled between the gate electrode of thefirst power saving transistor PSM1 and the first switching controller125A. The second switch S2 may be turned on in the power saving mode.The second switch S2 is in the turn-off state in the power saving mode.Accordingly, in the power saving mode, the first transistor M1 has theturn-off state, and only the first power saving transistor PSM1 mayperform a switching operation. The second switch S2 is in the turn-offstate in the normal mode.

The third switch S3 may be coupled between a gate electrode of thesecond transistor M2 and a gate electrode of the second power savingtransistor PSM2. The third switch S3 may be turned on the normal mode inresponse to the control signal supplied from the first switchingcontroller 125A. The third switch S3 has the turn-off state in the powersaving mode. Accordingly, both of the second transistor M2 and thesecond power saving transistor PSM2 may perform a switching operation inthe normal mode.

The fourth switch S4 may be coupled between the gate electrode of thesecond power saving transistor PSM2 and the first switching controller125A. The fourth switch S4 may be turned on in the power saving mode.The third switch S3 is in the turn-off state in the power saving mode.Accordingly, in the power saving mode, the second transistor M2 has theturn-off state, and only the second power saving transistor PSM2 mayperform a switching operation. The fourth switch S4 is in the turn-offstate in the normal mode.

In an embodiment, in the power saving mode, the first switchingcontroller 125A does not supply the control signal to the firsttransistor M1 and the second transistor M2.

In an embodiment, as described above, the first power voltage ELVDD isoutput by the first and second power saving transistor PSM1 and PSM2having small sizes in the power saving mode, so that the powerconsumption caused by the parasitic capacitance due to the switchingoperations of the first and second transistors M1 and M2 may be reduced.

FIG. 8 is a diagram illustrating an alternative exemplary embodiment ofthe second converter included in the DC-DC converter of FIG. 2 .

The second converter 140A of FIG. 8 may be substantially the same as thesecond converter of FIG. 5 , except for fifth to eighth switches S5 toS8. In FIG. 8 , components identical to those described with referenceto FIG. 5 are designated by like reference numerals, and any repetitivedetailed description thereof will be omitted.

Referring to FIGS. 5 and 8 , an embodiment of the second converter 140Amay include a second inductor L2, a third transistor M3, a third powersaving transistor PSM3, a fourth transistor M4, a fourth power savingtransistor PSM4, fifth to eighth switches S5, S6, S7 and S8, and asecond switching controller 145A.

The size of the third power saving transistor PSM3 may be smaller thanthat of the third transistor M3, and the size of the third power savingtransistor PSM3 may be smaller than that of the fourth transistor M4.

The fifth switch S5 may be coupled between a gate electrode of the thirdtransistor M3 and a gate electrode of the third power saving transistorPSM3. The fifth switch S5 may be turned on in the normal mode inresponse to a control signal supplied from the second switchingcontroller 145A. The fifth switch S5 is in the turn-off state in thepower saving mode. Accordingly, both of the third transistor M3 and thethird power saving transistor PSM3 may perform a switching operation inthe normal mode.

The sixth switch S6 may be coupled between the gate electrode of thethird power saving transistor PSM3 and the second switching controller145A. The sixth switch S6 may be turned on in the power saving mode. Thefifth switch S5 is in the turn-off state in the power saving mode.Accordingly, in the power saving mode, the third transistor M3 has theturn-off state, and only the third power saving transistor PSM3 canperform a switching operation. The sixth switch S6 is in the turn-offstate in the normal mode.

The seventh switch S7 may be coupled between a gate electrode of thefourth transistor M4 and a gate electrode of the fourth power savingtransistor PSM4. The seventh switch S7 may be turned on in the normalmode. The seventh switch S7 is in the turn-off state in the power savingmode. Accordingly, both of the fourth transistor M4 and the fourth powersaving transistor PSM4 may perform a switching operation in the normalmode.

The eighth switch S8 may be coupled between the gate electrode of thefourth power saving transistor PSM4 and the second switching controller145A. The eighth switch S8 may be turned on in the power saving mode.Accordingly, in the power saving mode, the fourth transistor M4 has theturn-off state, and only the fourth power saving transistor PSM4 canperform a switching operation.

In an embodiment, in the power saving mode, the second switchingcontroller 145A does not supply the control signal to the thirdtransistor M3 and the fourth transistor M4.

In such an embodiment, as described above, the second power voltageELVSS of which magnitude is adjusted by the third and fourth powersaving transistors PSM3 and PSM4 having small sizes is output in thepower saving mode, so that the power consumption caused by the parasiticcapacitance due to the switching operations of the third and fourthtransistors M3 and M4 may be reduced.

FIG. 9 is a waveform diagram illustrating an exemplary embodiment of anoperation of the DC-DC converter of FIG. 2 .

Referring to FIGS. 2 and 9 , an embodiment of the DC-DC converter 100may operate in the power saving mode during an enable period of the modecontrol signal PS_EN, and operate in the normal mode during a disableperiod of the mode control signal PS_EN.

In the normal mode, the display device may display a normal image. Inthe power saving mode, the display device may display an AOD image,e.g., an image such as a clock on a screen.

In an embodiment, the mode control signal PS_EN may perform transitionin a vertical blank period VBLNK. In addition, the enable period of themode control signal PS_EN may correspond to the power saving mode. In anembodiment, the disable period of the mode control signal PS_EN maycorrespond to the normal mode. The vertical blank period VBLNK is ablack period, and may overlap with a vertical synchronization signalVsync.

The driving manners of the first and second converters 120 and 140 maybe switched in the vertical blank period VBLNK. In one embodiment, forexample, when the mode of the display device is switched from the normalmode to the power saving mode, the driving manner of the first converter120 may be switched from the first driving manner to the second drivingmanner, and the driving manner of the second converter 140 may beswitched from the first or second driving manner to the third drivingmanner. In such an embodiment, when the mode of the display device isswitched from the power saving mode to the normal mode, the drivingmanner of the first converter 120 may be switched from the seconddriving manner to the first driving manner, and the driving manner ofthe second converter 140 may be switched from the third driving mannerto the first or second driving manner. In an embodiment, the switchingof the driving manners of the first and second converters 120 and 140may be performed in synchronization with the mode control signal PS_EN.In such an embodiment, the magnitude of the second power voltage ELVSSmay also be changed in the vertical blank period VBLNK. In anembodiment, the second power voltage ELVSS may be changed insynchronization with the mode control signal PS_EN.

Therefore, the magnitude of the second power voltage ELVSS and thedriving manner may be changed together in the vertical blank periodVBLNK. The magnitude of the second power voltage ELVSS in the powersaving mode may be larger than that of the second power voltage ELVSS inthe normal mode.

Accordingly, in such an embodiment, mode switching and fast switching ofdriving manners are implemented without using any black image periodthat occupies a separate image frame.

FIG. 10 is a waveform diagram illustrating an alternative exemplaryembodiment of the operation of the DC-DC converter of FIG. 2 .

In FIG. 10 , components identical to those described with reference toFIG. 9 are designated by like reference numerals, and any repetitivedetailed description thereof will be omitted.

Referring to FIGS. 2 and 10 , the enable period of the mode controlsignal PS_EN may correspond to the power saving mode, and the disableperiod of the mode control signal PS_EN may correspond to the normalmode.

The switching of driving manners of the first and second converters 120and 140 may be performed in synchronization with the mode control signalPS_EN. In one embodiment, for example, when the level of the modecontrol signal PS_EN is changed from a disable level to an enable level,the driving manner of the second converter 140 may be switched to thePSM manner. In such an embodiment, when the level of the mode controlsignal PS_EN is changed from the enable level to the disable level, thedriving manner of the second converter 140 may be switched to the DCM orCCM manner.

When the mode of the display device is switched from the normal mode tothe power saving mode, the magnitude of the second power voltage ELVSSmay be changed step by step to a power saving mode target levelthroughout a plurality of vertical blank periods VBLNK before theswitching of driving manners of the first and second converters 120 and140. In one embodiment, for example, the power saving mode target levelmay be above -to 2.5 V as shown in FIG. 10 . In such an embodiment, thesecond power voltage ELVSS and the luminance of the display panel may begradually and gently changed before the display device enters into thepower saving mode.

In such an embodiment, when the mode of the display device is switchedfrom the power saving mode to the normal mode, the magnitude of thesecond power voltage ELVSS may be changed step by step to a normal modetarget level throughout a plurality of vertical blank periods VBLNKafter the switching of driving manners of the first and secondconverters 120 and 140. In one embodiment, for example, the normal modetarget level may be about −4.2 V. In such an embodiment, the secondpower voltage ELVSS and the luminance of the display panel may begradually and gently changed before the display device enters into thenormal mode.

However, this is merely exemplary, and alternatively, the second powervoltage ELVSS and the luminance of the display panel may be graduallygently changed only in one of before the display device enters into thepower saving mode and before the display device enters into the normalmode.

In such an embodiment, as described above, the second power voltageELVSS is effectively prevented from being rapidly changed in modeswitching, so that a display defect such as flickering may be removed.

FIG. 11 is a waveform diagram illustrating another alternative exemplaryof the operation of the DC-DC converter of FIG. 2 .

In FIG. 11 , components identical to those described with reference toFIG. 9 are designated by like reference numerals, and any repetitivedetailed description thereof will be omitted.

Referring to FIGS. 2 and 11 , the enable period of the mode controlsignal PS_EN may correspond to the power saving mode, and the disableperiod of the mode control signal PS_EN may correspond to the normalmode.

In an embodiment, the mode switching of the DC-DC converter may beperformed in a black image period. The black image period is a period inwhich a black image is displayed as black data is applied to the displaydevice. In one embodiment, for example, the black image period maycorrespond to one frame period. However, this is merely exemplary, andalternatively, the black image period may correspond to a plurality offrame periods.

The mode selector 170 may allow the mode control signal PS_EN to beenabled in the black image period. The second converter 140 may changethe magnitude of the second power voltage ELVSS in the black imageperiod, and the driving manner of the second converter 140 may beswitched in synchronization with the mode control signal PS_EN in theblack image period. In an embodiment, the driving manner of the firstconverter 120 may also be switched in synchronization with the modecontrol signal PS_EN.

The black image period may be inserted into at least one of a period inwhich the mode of the display device is switched from the normal mode tothe power saving mode and a period in which the mode of the displaydevice is switched from the power saving mode to the normal mode.

In an embodiment, a short-circuit detection and shutdown operation ofthe DC-DC converter may be additionally performed in the black imageperiod.

FIG. 12 is a diagram illustrating a DC-DC converter according to analternative embodiment of the disclosure. FIG. 13 is a waveform diagramillustrating an exemplary embodiment of an operation of the DC-DCconverter of FIG. 12 . FIGS. 14A and 14B are waveform diagramsillustrating alternative exemplary embodiments of the operation of theDC-DC converter of FIG. 12 .

The DC-DC converter 100A of FIG. 12 may be substantially the same as theDC-DC converter of FIG. 2 , except fora protector 180. In FIG. 12 ,components identical to those described with reference to FIG. 2 aredesignated by like reference numerals, and any repetitive detaileddescription thereof will be omitted.

Referring to FIGS. 2 and 12 to 14B, an embodiment of the DC-DC converter100A may include a first converter 120, a second converter 140, a modeselector 170, and the protector 180.

The first converter 120 may output the first power voltage ELVDD in thefirst driving manner in the normal mode, and output the first powervoltage ELVDD in the second driving manner in the power saving mode.

The second converter may output the second power voltage ELVSS in one ofthe first to third driving manners in the normal mode, and output thesecond power voltage ELVSS in the third driving manner in the powersaving mode.

The mode selector 170 may provide the mode control signal PS_EN to thefirst and second converters 120 and 140 such that the first and secondconverters 120 and 140 are driven in the normal mode or the power savingmode, based on the external command CMD.

The protector 180 may perform a short-circuit detection operation, basedon a short-circuit detection signal SDET. The protector 180 may comparea variation in the first power voltage ELVDD output from the firstconverter 120 with a preset reference voltage VREF during a black imageperiod BIP in which the driving mode is switched between the normal modeand the power saving mode. In one embodiment, for example, the protector180 may perform the short-circuit detection operation during a detectionperiod in which the short-circuit detection signal SDET is enabled inthe black image period BIP. When the variation in the first powervoltage ELVDD is larger than the reference voltage VREF, the protector180 may output a shutdown signal SSD. The DC-DC converter 100A may beshut down by the shutdown signal SSD.

In an embodiment, as shown in FIG. 13 , the switching operations of thefirst converter 120 and the second converter 140 may be stopped in thedetection period DP of the black image period BIP. Accordingly, theoutput end of the first converter 120 and the output end of the secondconverter 140 may be floated. That is, the output end of the firstconverter 120 and the output end of the second converter 140 become ahigh impedance (“Hi-Z”) state.

At this time, in a normal case, the magnitude of the first power voltageELVDD is not smaller than a voltage variation set to a reference voltageVREF (e.g., a first reference voltage VREF1 in FIG. 13 ) due to avoltage stored in the first capacitor C1 coupled to the output end ofthe first converter 120.

However, when a short circuit occurs in the display panel or the DC-DCconverter 100A, the variation in the first power voltage ELVDD may begreater than that in the first reference voltage VREF1. At this time,the protector 180 may output the shutdown signal SSD.

However, this is merely exemplary, and alternatively, the protector 180may be coupled to the output end of the second converter 140 to detectthe short circuit of a circuit by sensing a change in the second powervoltage ELVSS. In one embodiment, for example, when a line fortransferring the second power voltage ELVSS and a line for transferringthe first power voltage ELVDD are short-circuited with each other, thesecond power voltage ELVSS may be considerably increased. In such anembodiment, as shown in FIG. 13 , the protector 180 may detect a shortcircuit by sensing a change in the second power voltage ELVSS andcomparing the sensed change in the second power voltage ELVSS with asecond reference voltage VREF2.

In an embodiment, the operation of the second converter 140 may bestopped in the black image period BIP. The protector 180 may be coupledto the output end of the second converter 140 to sense a change in thesecond power voltage ELVSS and compare the sensed change in the secondpower voltage ELVSS with the reference voltage (VREF of FIGS. 14A and14B). In one embodiment, for example, the operation of only the secondconverter 140 may be stopped in the detection period DP of the blackimage period BIP. Therefore, the output end of the second converter 140may be floated.

In the normal case, as shown in FIG. 14B, the second power voltage ELVSSmay not be allowed to be larger than the reference voltage VREF.Accordingly, the second power voltage ELVSS may be normally output inthe power saving mode after the black image period BIP.

As shown in FIG. 14A, when a short circuit occurs, the second powervoltage ELVSS may exceed the reference voltage VREF. At this time, theDC-DC converter 100A may be shut down.

In an embodiment, as described above, the black image period BIP isinserted just before the display device enters into the power savingmode from the normal mode, so that a short-circuit test may be performedduring the black image period BIP.

FIG. 15 is a diagram illustrating a DC-DC converter according to anotheralternative embodiment of the disclosure. FIG. 16 is a diagramillustrating an exemplary embodiment of an inverting converter moduleincluded in the DC-DC converter of FIG. 15 .

The DC-DC converter 200 of FIGS. 15 and 16 may be substantially the sameas the DC-DC converter of FIG. 2 , except for an inverting convertermodule 230. In FIGS. 15 and 16 , components identical to those describedwith reference to FIGS. 2 and 5 are designated by like referencenumerals, and any repetitive detailed description thereof will beomitted.

Referring to FIGS. 15 and 16 , an embodiment of the DC-DC converter 200may include a boost converter 220, the inverting converter module 230,and a mode selector 270. The inverting converter module 230 may includea first inverting converter 240 and a second inverting converter 250.

The boost converter 220 may correspond to one of the embodiments of thefirst converter 120 and 120A described above with reference to FIGS. 2to 12 . The boost converter 220 may output the first power voltage ELVDDin the first driving manner (e.g., the CCM manner) in the normal mode,and output the first power voltage ELVDD in the second driving manner(e.g., the DCM manner) in the power saving mode.

The first inverting converter 240 may correspond to one of theembodiments of the second converter 140 and 140A described above withreference to FIGS. 2 to 12 . The first inverting converter 240 mayoutput the second power voltage ELVSS in the third driving manner in thepower saving mode, and output the second power voltage ELVSS in one ofthe first, second and third driving manners based on the magnitude of aload of the display panel in the normal mode. In an embodiment, as shownin FIG. 16 , the first inverting converter 240 may have a configurationsubstantially identical to that of the second converter 140A of FIG. 8 .

The inverting converter module 230 may further include a phasecontroller for determining whether the second inverting converter 250 isto be driven, based on a load I_PANEL of the display panel, and aswitching controller 234 for controlling driving of the first invertingconverter 240 and the second inverting converter 250, based on an outputDUAL_EN of the phase controller 238 and the mode control signal PS_EN.

In an embodiment, the second inverting converter 250 does not operate inthe power saving mode. In such an embodiment, the second invertingconverter 250 may output the second power voltage ELVSS in the firstdriving manner or the second driving manner in the normal mode only whenthe load I_PANEL of the display panel exceeds a preset reference load.In such an embodiment, the second inverting converter 250 along with thefirst inverting converter 240 may output the second power voltage ELVSSonly when a high-luminance image having a preset level or higher isdisplayed.

Accordingly, only the first inverting converter 240 outputs the secondpower voltage ELVSS with respect to low luminance to middle luminance inthe normal mode, so that the power consumption of the DC-DC converter200 may be reduced.

In embodiments of a DC-DC converter and a display device including theDC-DC converter according to the disclosure, power voltages are outputusing a change in driving manner in the DC-DC converter and a change insecond power voltage according to driving manners, so that the powerconsumption caused by switching may be reduced. Thus, in suchembodiments, the heat generation and power consumption of the DC-DCconverter and the display device including the DC-DC converter may bereduced, and the conversion efficiency of the power voltages may beimproved.

Further, in such embodiments of the DC-DC converter and the displaydevice including the DC-DC converter according to the disclosure, thedriving manner and the second power voltage between the normal mode andthe power saving mode may be relatively freely controlled without anyadditional component. Thus, the cost to realize the power saving mode(e.g., the AOD mode) may be reduced, and the luminance in the powersaving mode having a level higher than that of the existing power savingmode may be implemented under an external environment in which ambientillumination is bright. In such embodiments, the driving manner of theDC-DC converter and/or the adjustment of the magnitude of the secondpower voltage may be relatively easily performed. Accordingly, themaximum luminance in the power saving mode may be flexibly controlled.

The invention should not be construed as being limited to the exemplaryembodiments set forth herein. Rather, these exemplary embodiments areprovided so that this disclosure will be thorough and complete and willfully convey the concept of the invention to those skilled in the art.In some instances, as would be apparent to one of ordinary skill in theart as of the filing of the application, features, characteristics,and/or elements described in connection with a particular embodiment maybe used singly or in combination with features, characteristics, and/orelements described in connection with other embodiments unless otherwisespecifically indicated.

While the invention has been particularly shown and described withreference to exemplary embodiments thereof, it will be understood bythose of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit or scopeof the invention as defined by the following claims.

What is claimed is:
 1. A direct current-to-direct current convertercomprising: a first converter which outputs a first power voltage in anormal mode or in a power saving mode based on a first inductor currentgenerated by alternately turning on a plurality of transistors therein,wherein the first converter operates in a first driving manner in thenormal mode, and operates in a second driving manner in the power savingmode; a second converter which outputs a second power voltage based on asecond inductor current generated by alternately turning on a pluralityof transistors therein, wherein the second converter operates in a thirddriving manner in the power saving mode; and a mode selector whichsupplies a mode control signal to the first and second converters,wherein the first and second converters are driven in the normal mode orthe power saving mode based on the mode control signal, wherein amagnitude of the second power voltage is changed step by step to a powersaving mode target level throughout a plurality of vertical blankperiods before a driving mode is switched from the normal mode to thepower saving mode.
 2. The direct current-to-direct current converter ofclaim 1, wherein turn-on times of the transistors in the second drivingmanner are less than turn-on times of the transistors in the firstdriving manner during a same period, and turn-on times of thetransistors in the third driving manner are less than the turn-on timesof the transistors in the second driving manner during the same period.3. The direct current-to-direct current converter of claim 1, whereinthe second converter operates in one of the first driving manner, thesecond driving manner and the third driving manner, which is determinedbased on a magnitude of a load of a display panel connected thereto, inthe normal mode.
 4. The direct current-to-direct current converter ofclaim 3, wherein the third driving manner is implemented as a pulsefrequency modulation manner.
 5. The direct current-to-direct currentconverter of claim 3, wherein when the first converter operates in thefirst driving manner, a magnitude of the first inductor current iscontinuously changed, and when the first converter operates in thesecond driving manner, the magnitude of the first inductor current isnot changed during a predetermined period.
 6. The directcurrent-to-direct current converter of claim 5, wherein when the secondconverter operates in the second driving manner, a magnitude of thesecond inductor current is not changed during a first discontinuousperiod, when the second converter operates in the third driving manner,the magnitude of the second inductor current is not changed during asecond discontinuous period, and the second discontinuous period islonger than the first discontinuous period.
 7. The directcurrent-to-direct current converter of claim 1, wherein the firstinductor current generated in the first driving manner has a first peakvalue, and to the first inductor current generated in the second drivingmanner has a second peak value less than the first peak value.
 8. Thedirect current-to-direct current converter of claim 1, wherein a drivingfrequency in the power saving mode is lower than a driving frequency inthe normal mode.
 9. The direct current-to-direct current converter ofclaim 8, wherein the driving frequency in the power saving mode isdecreased to a preset value as a magnitude of a load of a display panelconnected thereto is decreased.
 10. The direct current-to-direct currentconverter of claim 1, wherein the first converter includes: a firstinductor coupled between an input power source and a first node, whereinthe first inductor generates the first inductor current; a firsttransistor coupled between the first node and a ground; a first powersaving transistor coupled in parallel to the first transistor, whereinthe first power saving transistor has a size smaller than a size of thefirst transistor; a second transistor coupled between the first node anda first output end; a second power saving transistor coupled in parallelto the second transistor, wherein the second power saving transistor hasa size smaller than a size of the second transistor; and a firstswitching controller which controls on/off operations of the firsttransistor, the second transistor, the first power saving transistor andthe second power saving transistor, wherein the first transistor and thesecond transistor are alternately turned on in the normal mode, and thefirst power saving transistor and the second power saving transistor arealternately turned on in the power saving mode.
 11. The directcurrent-to-direct current converter of claim 10, wherein the firstconverter further includes: a first switch coupled between a gateelectrode of the first transistor and a gate electrode of the firstpower saving transistor, wherein the first switch is turned on in thenormal mode; a second switch coupled between the gate electrode of thefirst power saving transistor and the first switching controller,wherein the second switch is turned on in the power saving mode; a thirdswitch coupled between a gate electrode of the second transistor and agate electrode of the second power saving transistor, wherein the thirdswitch is turned on in the normal mode; and a fourth switch coupledbetween the gate electrode of the second power saving transistor and thefirst switching controller, wherein the fourth switch is turned on inthe power saving mode.
 12. The direct current-to-direct currentconverter of claim 11, wherein the first power saving transistor and thesecond power saving transistor are alternately turned on in the normalmode, in which the first and third switches are turned on.
 13. Thedirect current-to-direct current converter of claim 1, wherein thesecond converter includes: a second inductor coupled between a secondnode and a ground, wherein the second inductor generates the secondinductor current; a third transistor coupled between an input powersource and the second node; a third power saving transistor coupled inparallel to the third transistor, wherein the third power savingtransistor has a size smaller than a size of the third transistor; afourth transistor coupled between the second node and a second outputend; a fourth power saving transistor coupled in parallel to the fourthtransistor, wherein the fourth power saving transistor has a sizesmaller than a size of the fourth transistor; and a second switchingcontroller which controls on/off operations of the third transistor, thefourth transistor, the third power saving transistor and the fourthpower saving transistor, wherein the third transistor and the fourthtransistor are alternately turned on in the normal mode, and the thirdpower saving transistor and the fourth power saving transistor arealternately turned on in the power saving mode.
 14. The directcurrent-to-direct current converter of claim 13, wherein the secondconverter further includes: a fifth switch coupled between a gateelectrode of the third transistor and a gate electrode of the thirdpower saving transistor, wherein the fifth switch is turned on in thenormal mode; a sixth switch coupled between the gate electrode of thethird power saving transistor and the second switching controller,wherein the sixth switch is turned on in the power saving mode; aseventh switch coupled between a gate electrode of the fourth transistorand a gate electrode of the fourth power saving transistor, wherein theseventh switch is turned on in the normal mode; and an eighth switchcoupled between the gate electrode of the fourth power saving transistorand the second switching controller, wherein the eighth switch is turnedon in the power saving mode.
 15. The direct current-to-direct currentconverter of claim 14, wherein the third power saving transistor and thefourth power saving transistor are alternately turned on in the normalmode, in which the fifth and seventh switches are turned on.
 16. Thedirect current-to-direct current converter of claim 1, wherein thesecond converter switches a driving manner thereof in synchronizationwith the mode control signal in a vertical blank period, and themagnitude of the second power voltage is changed step by step to anormal mode target level throughout a plurality of vertical blankperiods after the driving mode is switched from the power saving mode tothe normal mode.
 17. The direct current-to-direct current converter ofclaim 1, wherein the mode selector allows the mode control signal to beenabled in a black image period, and the second converter switches adriving manner thereof in synchronization with the mode control signal.18. The direct current-to-direct current converter of claim 1, furthercomprising: a protector which compares a variation in the first powervoltage output from the first converter with a preset reference voltageduring a black image period in which the driving mode is switchedbetween the normal mode and the power saving mode, and shuts down thedirect current-to-direct current converter if the variation in the firstpower voltage is greater than the reference voltage.
 19. The directcurrent-to-direct current converter of claim 18, wherein switchingoperations of the first and second converters are stopped in the blackimage period, and the protector compares the variation in the firstpower voltage with the reference voltage in a state in which an outputend of the first converter and an output end of the second converter arefloated in the black image period.
 20. The direct current-to-directcurrent converter of claim 1, further comprising: a protector whichcompares the second power voltage output from the second converter witha preset reference voltage during a black image period in which thedriving mode is switched between the normal mode and the power savingmode, and shuts down the direct current-to-direct current converter ifthe magnitude of the second power voltage is greater than the referencevoltage, wherein an operation of the second converter is stopped duringat least a portion of the black image period.